Fly ash comprises finely divided inorganic products produced by the combustion of coal. Enormous amounts of fly ash are produced annually nationwide, principally from burning coal in electric power plants. Disposal of fly ash poses an increasingly difficult problem because the volume, the particulate nature and the varied chemical composition of fly ash limit the number of acceptable disposal sites. Such sites require sophisticated and expensive engineering, design, construction and operational controls to be in place to manage and dispose of the fly ash. Furthermore, the capacity of these disposal sites is not unlimited.
Fly ash of particular composition has been used as an additive in portland cement but such fly ashes have not been hydraulic, i.e., self-setting. Rather, the fly ash has been added to strengthen the cement, evidently, by reacting with excess free lime and by reacting with sulfate compounds which would otherwise attack the tricalcium aluminate and tetracalcium aluminoferrite compounds of portland cement.
U.S. Pat. No. 4,997,484 to Gravitt et al. discloses a hydraulic cement which uses a subbituminous fly ash to achieve high strength in a short time when cured at room temperature. However, the cement compositions disclosed in this patent have limited uses because they cure too quickly. Particularly, the cement compositions disclosed in Gravitt initially cure in less than thirty minutes. Such a short period of time for curing is too short to use the cement compositions for anything other than patching.
Another quick curing hydraulic cement composition is disclosed in U.S. Pat. No. 5,374,308 to Kirkpatrick et al. The compositions disclosed in the Kirkpatrick patent employ a class C fly ash to achieve high strength in a short period time. Like the compositions disclosed in Gravitt, the compositions disclosed in Kirkpatrick cure too quickly to be useful for any other purpose than patching.
Thus, a need has developed in the art for a blended hydraulic cement formulated with fly ash which can be used for a variety of purposes from patching to forming concrete objects.